Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO₂ lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO₂) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

沸石负载的钼作为甲烷脱氢芳构化的催化剂已被广泛研究。尽管取得了重大进展，但实际的中间体，特别是第一个 CC 键的形成尚未得到阐明。在此，我们报告了钼单位点上非氧化甲烷活化过程中甲基自由基的演化，这选择性地产生了增值化学品。Operando X射线吸收光谱和在线同步加速器真空紫外光电离质谱结合电子显微镜和密度泛函理论计算揭示了钼单中心在甲基自由基生成中的重要作用，并且甲基自由基的生成速率与钼单位点的数量。甲基自由基在气相中转化为乙烷，在没有沸石的情况下很容易脱氢为乙烯。这基本上类似于先前报道的SiO ₂晶格限制单中心铁催化剂的反应途径。然而，沸石的可用性，无论是物理混合物还是作为载体，都会引导随后的反应途径在沸石限制的孔内进行芳构化，从而导致苯作为主要的烃产物。研究结果表明，无论载体如何（沸石 MCM-22 和 ZSM-5 或 SiO ₂），甲基自由基化学可能是金属单中心催化的一般特征，而聚集的碳化钼纳米粒子上的反应可能促进通过表面 CC 的碳沉积耦合。这些发现进一步加深了对非氧化甲烷转化为增值化学品的基本认识。